Catalysts are employed in a number of processes ranging from refining crude oil to treating waste streams. These catalysts usually contain one or more catalytic metals deposited on a support which has a high surface area and high porosity. These support properties are necessary in order to have a catalyst with high activity and good durability. The support is generally a refractory inorganic oxide which may be utilized in a number of configurations, shapes and sizes. For example the support may be formed in the shape of spheres, extrudates, irregularly shaped granules, etc. or the support may be deposited as a layer onto a rigid structure such as a metal or ceramic honeycomb structure.
The use of a honeycomb structure has been the preferred configuration in treating exhaust gases, e.g., from automotive engines, because it reduces the weight of the catalyst and minimizes the amount of back pressure on the engine. Typically, a honeycomb structure composed of a polycrystalline cordierite phase which has a surface area less than 1 m.sup.2 /g is used. Cordierite is used because of its good strength and thermal shock resistance. However, treating an exhaust stream with a catalyst consisting of a cordierite monolithic structure which has been coated with a support containing catalytic metals does have some drawbacks. One drawback is that the thermal expansion coefficient of cordierite is different than that of the support. Therefore, during thermal cycling some of the support, which contains the catalytic metals, can flake off and be carried off in the exhaust stream. This deteriorates the activity of the catalyst. The exhaust gases also contain dust or particulate matter which can also cause the high surface area support to flake off and be carried off. These problems could be eliminated if the monolithic structure had a high surface area such that the catalytic metals could be deposited on the honeycomb structure without using a support. Eliminating the support coating would also represent an economic advantage because a processing step would be eliminated.
This problem has received attention in the art and some solutions have been proposed. However, the solutions have centered on adding a high surface area porous oxide phase to the ceramic phase, e.g., cordierite. For example, U.S. Pat. No. 4,631,268 to Lachman et al. discloses a monolithic structure having a substantially continuous high strength ceramic phase selected from the group consisting of cordierite, mullite, clay, talc, zirconia, zirconia-spinel, alumina, silica, lithium aluminosilicates, alumina-zirconia and mixtures thereof and a discontinuous high surface area porous phase selected from alumina, silica, spinel, titania, zirconia, zeolite, and mixtures thereof.
U.S. Pat. Nos. 4,657,880 and 4,637,995 disclose similar materials with minor modifications. In all three of these patents, the high surface area is contributed by a porous oxide. Additionally, U.S. Pat. No. 4,631,267 discloses a high surface area monolithic structure composed of a high surface area porous oxide phase and a permanent binder. The porous oxide phase is selected from the group consisting of alumina, silica, zeolite, and spinels. The above patents disclose and claim that the resulting monolithic structures can be used as catalyst supports without the use of a separate support layer deposited on the monolithic structure.
In contrast to the solutions provided by the prior art, applicants' solution to the problem is a catalyst support structure comprising a substantially polycrystalline cordierite phase having a surface area of at least 2.7 m.sup.2 /g and preferably at least 8 m.sup.2 /g. Applicants are the first to produce a high surface area cordierite catalyst support structure. There is nothing in any of the above cited references that even hints that a high surface area cordierite monolithic structure can be formed.
The instant application also relates to a method of producing a high surface area catalyst support structure. This process involves coprecipitating aluminum, magnesium and silicon compounds, followed by extrusion through a die to form a desired shape and heating the shape to form a high surface area cordierite catalyst support structure.
The prior art does not disclose that a high surface area catalyst support structure can be produced by the method described above. The closest prior art shows that a high density, low surface area (opposite properties of applicants' product) can be prepared by coprecipitating aluminum, magnesium and silicon compounds, forming the powder into a shape and sintering at high temperatures. Such a procedure is described in "Synthesis of Oxide Ceramic Powders by Aqueous Coprecipitation" by J. R. Moyer et al., Materials Research Society Symposium, Vol. 73, p. 117 (1986). The aim of the synthesis in the Moyer reference is to form ceramic products which have a density of about 98% of its theoretical density, whereas applicants' product has at least 20% porosity (low density). Thus, the Moyer reference neither suggests nor hints at a method of producing a high surface area catalyst support structure. It is applicants alone who have discovered such a method.